The present invention relates to surveys in a subterranean borehole for determining the spatial co-ordinates of its path. More particularly, the present invention relates to determining the spatial coordinates and hence the azimuth of a borehole using accelerometers.
Within the prior art accelerometers have been used for determining gravity at a particular point (G), inclination, rotation and the horizontal plane. Frequently magnetometers or gyroscopes combined with the accelerometers are used to determine direction. There are situations when magnetometers are affected by sources of magnetic interference like nearby magnetic steel, electromagnetic radiation and ferric minerals in formations or ore bodies. The main cause of concern from these sources is the deflection of the Azimuth readings obtained from the magnetometers, which the magnetic interference can cause. The Azimuth is also affected by the so-called xe2x80x9cDrill string interferencexe2x80x9d. Although it is called drill string interference the cause of this magnetism is mainly from motors and stabilisers. Motors and stabilisers often are permanently magnetized during magnetic particle inspection processes. While the motors and stabilisers are generally degaussed following inspection, the degaussing process is frequently inadequate, resulting in accumulation of magnetic interference from use to use.
As mentioned above, gyroscopes have also been used as surveying instruments in the prior art. Gyroscopes can be considered to be more complex instruments than the others mentioned and due to increasing time dependent errors frequently have to be re-referenced and protected from high temperatures and vibration. Furthermore, gyroscopes possess a significant disadvantage in that at low angles of inclination the azimuth is variable. At higher inclination angles this effect stabilises. A consequence is that gyro""s cannot give direction or tool face direction at low inclinations. To kick a well off at low inclination in a specific direction, is not possible with a gyro. A gyro needs a few degrees inclination in the hole before it can determine the well and tool face direction. When the well builds to an angle that the gyro""s can use it may be in the wrong direction. This then has to be corrected and wastes valuable steer-able footage.
In order to overcome the above described problems of the prior art borehole surveying techniques, it is an object of the present invention to provide the Inclination and Azimuth of a borehole without the use of magnetometers or gyroscopes by instead using accelerometers. As accelerometers are responsive to the Earth""s gravity they are immune to the sources of interference which affect magnetometers and gyroscopes, and hence the use of accelerometers frees the measurement system from the constraints of these devices. The result is a less complex, more rugged apparatus for determining the borehole position.
According to the present invention, there is provided a method of surveying a borehole to determine at least the inclination and azimuth of said borehole at one or more survey positions along said borehole, comprising the steps of:
a) aligning at least one of a first or second set of gravity measurement means with a reference azimuth;
b) moving said first and second sets of gravity measurement means along said borehole until said first set rests at a survey position and said second set rests at another position, the movement being such that a rotational orientation between said first and second sets of gravity measurement means about a first axis along said borehole is maintained;
c) measuring a first set of two or more gravity vectors at said first survey position with said first set of gravity measurement means, said first set of gravity vectors being mutually perpendicular;
d) measuring a second set of two or more gravity vectors at said other position with said second set of gravity measurement means, said second set of gravity vectors being mutually perpendicular; and
e) calculating the inclination and azimuth of said borehole at said first survey position from said first and second sets of gravity vector measurements;
wherein steps b) to e) may be repeated at said one or more survey positions such that the borehole may be surveyed along the length of the borehole.
Furthermore, the present invention also provides an apparatus for surveying a borehole to determine at least the inclination and azimuth of said borehole at one or more survey positions along said borehole, comprising:
a first set of gravity measurement means arranged to measure a first set of two or more gravity vectors, said first set of gravity vectors being mutually perpendicular;
a second set of gravity measurement means arranged to measure a second set of two or more gravity vectors, said second set of gravity vectors being mutually perpendicular;
a joining structure arranged to join said first and second set of gravity measurement means to prevent any relative rotation therebetween; and
processing means arranged to calculate the azimuth and inclination of the borehole at the survey position from the gravity vectors measured by the first and second set of gravity measurement means;
wherein said apparatus is further arranged so as to permit movement of said apparatus along said borehole, a long axis of said joining structure being co-axial with said borehole along the length of said borehole.
Each set of gravity measurement means may measure two or preferably three mutually perpendicular gravity vectors. Where only two gravity vectors are measured a corresponding third gravity vector for each set is found by a consideration of the known local total gravitational field, and solving for the unknown third vector from this known local total value.
With respect to alignment of the gravity measurement means, the alignment may be performed before the gravity measurement means are run into the borehole, the alignment then being maintained as the means are run into said borehole. Alternatively, alignment of the means may be achieved by aligning at least one of the sets of gravity measurement means with a part of the borehole with a known azimuth. Either or both of the first and second sets of gravity measurement means may be aligned with the reference azimuth, although where only one of the sets is aligned, then the rotational offset between the two sets of gravity measurement means must be known. Preferably, there is no rotational offset between the two sets of gravity measurement means, and the two sets of means are rotationally aligned about long axis of the borehole.
The distance between the two sets of gravity measurement means defined by the joining structure may be constant, or may instead be variable. Where the distance is variable along the borehole it is preferable that the distance be known at all times. In a preferred embodiment, the first and second gravity measurement means each preferably comprise two or more mutually perpendicular accelerometers, each accelerometer being arranged to measure one of the gravity vectors of each respective set.
Furthermore, the present invention is particularly suitable for making dynamic measurements as the surveying tool is moved along the borehole, and it is not necessary for the surveying tool carrying the apparatus of the present invention to be stationary when measurements are taken.
The present invention has a primary advantage in that it is highly resilient to shock and changes in temperature and hence is suitable for use in applications where gyroscopic techniques cannot be relied upon.
Furthermore, a further advantage in that the method and apparatus of the present invention are also resistant to magnetic interference caused by magnetic minerals in the surrounding rock. The present invention may therefore replace magnetic survey techniques using magnetometers in areas where magnetic interference is a problem.
The use of accelerometers in the present invention refers to them being used to determine the inclination and direction of a borehole. Other sensors such as magnetometers and/or gyroscopes may be used in conjunction with the present invention to give other useful surveying information such as, for example, the positions of sources of interference along the borehole e.g. ore bodies, steel pipes, formation magnetic logs etc.